This invention generally relates to a new process for making monolithic multilayer ceramic capacitors (MLCs). The process involves a calendering step in which lumps or other imperfections are removed from the dried electrode print. This step enables use of thinner dielectric layers and the achievement of higher values of capacitance per unit part volume.
Multilayer ceramic capacitors consist of a plurality of interleaved and staggered layers of an electrically conductive film of metal (termed "electrode"), formed by the deposition (usually be screen printing or variations thereof) of a thick film paste (termed an "electrode composition") and electrically insulating layers of a ceramic oxide (termed "dielectric"), formed by laying a cast dielectric tape or by casting a dielectric slurry over the dried electrode. Such capacitors are well-known in the art. U.S. Pat. No. 2,389,420, for example, describes the structure, manufacture and properties of monolithic multilayer ceramic capacitors.
The electrode composition is usually a dispersion of finely divided precious metal powders such as palladium, silver, gold, or platinum or their mixtures in a vehicle which is usually solely organic in nature. Dispersions of non-precious metals such as copper and nickel have also been shown to have utility in electrode compositions. The vehicle is usually composed of a mixture of a polymeric resin which imparts viscosity to the composition and appropriate solvents for processing compatibility, particularly with respect to drying. Other organic additives are usually made to the vehicle to control paste rheology. Typical electrode composition metal concentrations range from 40 to 70% by weight, with the remainder being vehicle. Electrode compositions are deposited, usually be screen printing techniques, on dried dielectric layers, then dried to remove solvents and leave a mixture of metal powders and resin from the vehicle.
The dielectric layer is usually composed of finely-divided oxide powers dispersed in a resin. Barium titanate (BaTiO.sub.3) and other oxides such as neodymium titanate (Nd.sub.2 Ti.sub.2 O.sub.7) and magnesium titanate (MgTiO.sub.3) are used. Additions are usually made to these oxides to control various electrical characteristics, particularly to maximize dielectric constant while controlling the temperature dependence of dielectric constant and electrical insulation resistance, among other properties. The resin is present in the dielectric layers to facilitate handling and printing of electrodes on the layers.
Multilayer ceramic capacitors are manufactured by building up an interleaved configuration of electrode and dielectric layers, dicing individual parts out of the build-up then subjecting the parts to a slow burnout then high temperature firing. Burnout is done to remove the organic resin in the electrode and dielectric layers to avoid rapid outgassing and rupture of the parts. Firing is done to a peak temperature to both densify the dielectric for maximum dielectric constant and physical strength, and to react the chemical constituents of the dielectric such that other desired electrical characteristics are achieved. During the firing step, the powder grains in the electrode layers also sinter and densify so as to produce a continuous, highly electrically conductive metal film.
Multilayer ceramic chip capacitors are made using two primary manufacturing processes: the "dry stack" and "wet stack" processes. An excellent review of MLC manufacturing processes is given in a paper by Adair et al., "A Review of the Processing of Electronic Ceramics with Emphasis on Multilayer Ceramic Capacitor Fabrication" in Journal of Materials Education, Vol. 9, pages 71-118 (1987). Both processes relate to the manner in which the dielectric layer is formed over the electrode lying underneath it. In the dry stack process the dielectric is cast into a tape from a slurry of dielectric powder, solvents, and pliable resins. The tape is placed over the dried electrode print, a subsequent electrode is printed on the tape, dried, and another tape is placed, etc. The dry stack process is limited in the thinness of the fired dielectric layer which can be achieved since the tape must be of sufficient thickness for handling. The "wet stack" process utilizes slurries of dielectric powder, solvents and resins, but involves casting, printing, or pouring the dielectric slurry directly over the dried electrode. The wet stack process is inherently capable of forming thinner dielectric layers since the requirements of tape handling are avoided.
A principal trend in MLC industry, and in electronics in general, is towards smaller components with higher electrical values per unit part volume. There are several ways to achieve this goal in multilayer ceramic capacitors. One way is to utilize ceramic dielectrics with higher dielectric constants. This is, however, limited by fundamental limitations with the solid state chemistry of the dielectrics. Another method is to have the capacitors contain as many electrically active layers as possible. This is limited by both physical restraints in the allowable part thickness and the tendency for high layer count parts to delaminate during burnout and/or firing.
A third way to achieve higher values of capacitance per unit part volume is to make the dielectric layers thinner so as to reduce the distance between electrodes of opposite polarity. This is possible using the finer dielectric powders now available and conventional tape casting or wet stack technology. The principal limitation, however, is the smoothness of the printed electrode.
In current MLC electrode paste technology, it is not unusual for a dried electrode surface to have asperities or lumps of metal protruding above the bulk of the print. In some cases these lumps can extend up to 15 microns above the print surface. If a dielectric tape of 20 microns dried thickness is placed over such a lump, the lump may protrude through the tape and come into contact with the opposing electrode. Such contact will result in an electrical short in the capacitor. Even if interelectrode contacts as described do not immediately occur, long term part reliability may still be questionable.
Reliability is a prime concern in multilayer ceramic capacitors. Electronic designers require high degrees of reliability in every component comprising an electrical circuit. In particular, they rely heavily on the reliability of the simpler passive components such as capacitors being higher than other components to compensate for lower degrees of reliability in other components.
The dried electrode lump problem is exacerbated when the wet stack MLC manufacturing process is used. In this process the dielectric layer is applied in liquid form over the dried electrode. Any lumps or other imperfections existing in the dried electrode can easily poke through the wet layer and contact the electrode of opposite polarity on the other side of the cast dielectric. The dried electrode lump problem is particularly daunting in the case of the wet stack manufacturing process because this process facilitates making thin dielectric layers.
There are two possible means to eliminating the lump problem in dried electrodes. One is to optimize the process used to disperse the electrode metal powder in the electrode organic vehicle. Techniques such as three-roll-milling, sand-milling, and high speed dispersion are used. Such techniques, however, always have some degree of impact character associated with their dispersion action. Impact can cause the individual grains comprising the electrode powder to aggregate into larger grains which ultimately can become lumps in the printed and fired electrode. This is exacerbated when malleable precious metal powders are used, as is the case for the dispersion of Pd and Pd/Ag powders in MLC electrodes. Even when such dispersion processes are optimized to control powder grain aggregation, there always exists the probability that some powder aggregation will occur. With the high levels of reliability required of MLCs in the present, and in the future, such probabilities are unacceptable.
Another way to eliminate the lump problem is to eliminate, or at least control, the number and size of the lumps present in the powder in the electrode while the powder is being precipitated. Even with an optimized powder dispersion technique, any aggregates present in the starting powder will only be dispersed into the organic vehicle and not broken up into smaller sized grains. It is unlikely, however, that control of such lumps can be achieved using purely chemical techniques during powder precipitation.
Ultimately some physical (non-chemical) means of controlling electrode lumps is needed to enable use of thinner dielectric layers in multilayer ceramic capacitors.